How can we improve WHMCS?

We are faced with a Issue of Customers having High Outstanding Amount which they need to pay us. However The process of recovery of money is very painful and time consuming and so we request the below feature . Situation explained below :-

Assume a Customer has a Outstanding of eg 1000 Dollars ( from a Manual Activity done during Migration) . Then after the system comes online he keeps on renewing till he exhausts his credits to Zero Dollar. So the current Situation is
Outstanding 1000$ and Credit Zero Dollars.
In the current situation he will add eg 2000$ to his Account and he continues to renew his other Domains whereas "the Outstanding remains as it is at 1000$" which is a Loss for us . ( This should not be possible until he clears his previous Outstanding)

We would like to configure the system in such a way that Suppose he has a OutStanding of 1000$ and he puts in 2000$ then directly System should debit 1000$ (so we recover Previous Outstanding) and then give him a credit of 1000$ (2000-1000) for new renewals.
In another scenario in which he has a Outstanding of 1000$ and he puts in 500$ this should also get automatically debited and his Outstanding will now show OutStanding 500$ i.e he will not be able to do any new Renewals/Transactions unless he clears his Outstanding. Thus No New Activity unless previous Outstanding is cleared .This will be very useful for many WHMCS Customers and help in faster Money Recovery .
Can you pls help make this possible. Thanks .